Both of the Disney waterparks have an area specifically for younger kids. Wet n Wild does too.
I found the Disney park Typhoon Lagoon more relaxing than Wet N Wild. There are sandy beach areas for the kids to play in, slides, spouts etc, DD at 2 loved the lazy river tube ride they had there in the little ones area. Couldn't get her out of it! Lunch for the kids was served in a pail so they could use it on the beach afterwards. I also saw an ice cream sundae in one of those pails, with a shovel for your spoon!
Wet n Wild is more of a concrete park with thrill rides for the older kids.
I haven't tried Aquatica, so can't comment on that park (but I understand there is little shade, so you may want to bear that in mind with young kids)
--Kay